Typically, in the field of bicycles, the expression “right crank arm assembly” is used to indicate an assembly comprising a right crank arm and at least one front sprocket coupled with the right crank arm. The front sprocket, in particular, is the toothed wheel adapted to drive the chain of the bicycle for the motion transmission to the rear wheel of the bicycle, such motion being imparted by the cyclist through pedaling.
In conventional bicycles, the right crank arm is directly coupled with the front sprocket at respective surfaces defined on respective front faces of such components. In particular, defining as inner side of the crank arm and of the front sprocket the one intended to face, in use, towards the frame of the bicycle and as outer side the one opposite the inner side and intended to face, in use, towards the outside, the coupling between right crank arm and front sprocket typically takes place at one of the sides of the crank arm and at one of the sides of the front sprocket.
It has been observed that, in operation, the front sprocket elastically deforms due to the stresses to which it is subjected during pedaling. Such a deformation leads to a decrease in the transmission efficiency of the motion imparted through the crank arm assembly.
In particular, the crank arm transmits to the front sprocket the force that the cyclist exerts upon the pedals of the bicycle. Such a force acts in a plane parallel to the middle plane of the front sprocket and its direction of application and intensity change at each angular position of the pedal. This stress therefore causes a variable lateral flexing deformation of the front sprocket. Considering also that the chain exerts on a portion of the front sprocket a force opposing the forward movement of the front sprocket, the resulting stress on the front sprocket is a twisting stress. Therefore, the consequent deformation of the front sprocket is a twisting deformation.
Such a deformation occurs in all conventional front sprockets, even if they are made from metallic material, but it is rather accentuated in the case of front sprockets made from lightweight materials.
Indeed, it is known, above all in the field of racing bicycles, to use front sprockets made from light alloys, like for example aluminum alloys, and composite alloys, i.e. made partly from metallic material and partly from another material, like for example carbon fiber.
The right crank arm assemblies of the prior art typically comprise a star-shaped right crank arm. Such a crank arms comprise, in particular, a plurality of coupling arms having at respective free ends, a portion for fixing to the front sprocket.
The Applicant has found that, in order to reduce the overall weight of known assemblies to the minimum, the coupling arms of the crank arms and the corresponding coupling elements of the front sprockets are made with very low thickness, this causing the twisting of the front sprockets.
The Applicant has also found that, also in the case of strengthened crank arms, front sprockets made from light material still continue to bend a lot.